1. Field of the Invention
This invention relates generally to wireless communications and, more particularly, to the operation of a Radio Frequency (RF) receiver within a component of a wireless communication system.
2. Description of the Related Art
The structure and operation of wireless communication systems are generally known. Examples of such wireless communication systems include cellular systems and wireless local area networks, among others. Equipment that is deployed in these communication systems is typically built to support standardized operations, i.e., operating standards. These operating standards prescribe particular carrier frequencies, modulation types, baud rates, physical layer frame structures, MAC layer operations, link layer operations, etc. By complying with these operating standards, equipment interoperability is achieved.
In a cellular system, a regulatory body typically licenses a frequency spectrum for a corresponding geographic area (service area) that is used by a licensed system operator to provide wireless service within the service area. Based upon the licensed spectrum and the operating standards employed for the service area, the system operator deploys a plurality of carrier frequencies (channels) within the frequency spectrum that support the subscribers' subscriber units within the service area. Typically, these channels are equally spaced across the licensed spectrum. The separation between adjacent carriers is defined by the operating standards and is selected to maximize the capacity supported within the licensed spectrum without excessive interference. In most cases, severe limitations are placed upon the amount of adjacent channel interference that maybe caused by transmissions on a particular channel.
In cellular systems, a plurality of base stations is distributed across the service area. Each base station services wireless communications within a respective cell. Each cell may be further subdivided into a plurality of sectors. In many cellular systems, e.g., Global System for Mobile Communications (GSM) cellular systems, each base station supports forward link communications (from the base station to subscriber units) on a first set of carrier frequencies, and reverse link communications (from subscriber units to the base station) on a second set of carrier frequencies. The first set and second set of carrier frequencies supported by the base station are a subset of all of the carriers within the licensed frequency spectrum. In most, if not all, cellular systems, carrier frequencies are reused so that interference between base stations using the same carrier frequencies is minimized and system capacity is increased. Typically, base stations using the same carrier frequencies are geographically separated so that minimal interference results.
Both base stations and subscriber units include RF receivers. Radio frequency receivers service the wireless links between the base stations and subscriber units. The RF transmitter receives a baseband signal from a baseband processor, converts the baseband signal to an RF signal, and couples the RF signal to an antenna for transmission. In most RF transmitters, because of well-known limitations, the baseband signal is first converted to an Intermediate Frequency (IF) signal and then the IF signal is converted to the RF signal. Similarly, the RF receiver receives an RF signal, down converts the RF signal to an IF signal and then converts the IF signal to a baseband signal. In other systems, the received RF signal is converted directly to a baseband signal.
One problem in down converting a received RF or IF signal that particularly causes difficulty is that of intermodulation interference. More specifically, a single interference signal in an adjacent channel does not typically introduce a significant amount of interference because its effects may be filtered out or minimized. However, if a plurality of interference signals are present in adjacent channels, then the interaction of each of the interference signals may cumulate to create intermodulation interference in the present channel being used to receive a specified communication signal. Such interference is often referred to as a third order product and is desirably filtered to reduce or eliminate the effect upon the communication signals in the present channel.
There is a need in the art, therefore, for a low power RF receiver that provides gain level settings in a manner that reduces the effects of intermodulation interference.